In ball nut and screw assemblies, static axial loading on the nut and screw produces a torque tending to cause relative rotation of the nut and screw and thus their relative translation. If inefficiencies within the entire ball screw drive and support system (i.e. pulleys, gears, bearings, bushings, etc.) do not produce a resisting torque equal to or greater than this static axial load induced torque, the ball screw may "backdrive" and lose position. Many ball-less nut and screw assemblies having acme threads do not backdrive because of the inherently low internal efficiency. But this low efficiency produces undesirable stick/slip operation, premature wear of the nut and screw, higher power consumption and increased heat generation.
Various mechanisms have been proposed to provide braking in a ball nut and screw assembly but they are typically costly to manufacture, require considerable accommodating space and/or require modification of the ball screw. Moreover, they are either not adapted for or are not readily adjustable to compensate for wear and/or prevent backdriving at different load levels. For example, U.S. Pat. No. 4,938,090 discloses a preloaded ball nut and screw assembly that is said to withstand continual reciprocating motion but would not prevent backdrive to any significant degree, let alone to a controlled degree. U.S. Pat. No. 2,447,439 discloses a bulky self-locking mechanism that is both complex and without easy adjustment. U.S. Pat. No. 2,623,403 discloses a restraining mechanism that uses low efficiency acme threads on it and the screw, in addition to the normal ball receiving groove on the screw. This complicates and adds to the cost of the ball screw. Moreover, this restraining mechanism is not readily adjustable to compensate for wear and different load levels.